Laser-induced thermal desorption analysis of the surface during Ge etching in a Cl2 inductively coupled plasma

Abstract

Laser desorption laser-induced fluorescence (LD-LIF) detection of GeCl was used to determine in situ the surface coverage of chlorine during the etching of germanium by Cl2 in an inductively coupled plasma (ICP) reactor. The ICP operated in the dim mode for radio frequency (rf) power ≲350 W and in the bright mode for higher powers. The etch rate was 3.5 μm/min with 540 W rf power and −40 V substrate bias. The chlorine surface coverage was about 2× that with chlorine flow only and the plasma off, both with dim- and bright-mode operation, and was independent of rf power within each mode for laser repetition rates of 0.2, 5, and 15 Hz. Similarly, the chlorination of the adlayer did not change when the ion energy was increased from 16 to 116 eV by increasing the substrate bias voltage, both with dim- and bright-mode operation. This was confirmed by x-ray photoelectron spectroscopy measurements in a similar high density reactor, where it was found that the surface density of chlorine was ∼2.6×1015 Cl/cm2. As the ion energy increases from 16 to 116 eV, the etch yield of ions increases from 1 to 3 atoms/ion. “Wait and probe” measurements show that the GeClx adlayer is quite stable. Since the same adlayer chlorine content was measured by LD-LIF under high ion current/fast etch conditions (bright mode) and low ion current/slow etch conditions (dim mode), both the adlayer chlorine content and the etch rate seem to be controlled by the ion current to the wafer. Previously reported work in Si etching in this ICP [J. Vac. Sci. Technol. A 15, 3024 (1997)] showed a similar independence of adlayer chlorination with rf power, but much slower chlorination and an increasing chlorination of the adlayer with increasing ion energy.